WASHINGTON — A man who planned a mass shooting at the Washington headquarters of a conservative Christian lobbying group should spend 11½ years in prison for the failed plot, not the 45 years government lawyers want, his defense attorney says.

A lawyer for Floyd Corkins II made the recommendation in a document filed Tuesday.

Corkins’s lawyer says his client was mentally ill when he entered a building housing the Family Research Council armed with nearly 100 rounds of ammunition and 15 Chick-fil-A sandwiches and shot and wounded a security guard.

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Corkins later told authorities he planned to kill as many people as possible and smear the sandwiches on their faces as a political statement. Chick-fil-A was making headlines at the time because of its president’s stated opposition to gay marriage.

A judge is set to sentence the 29-year-old Corkins on Sept. 19. Prosecutors recommended he spend 45 years in prison. But Corkins’s lawyer disagreed in a memo filed Tuesday. He wrote that if Corkins were ‘‘unrepentant and . . . not suffering from a mental illness at the time he committed the offenses, a severe sentence might indeed be warranted.’’